This invention relates to a comparison type dimension measuring method using a laser beam in a microscope system in which the dimensions of a fine pattern such as an IC pattern which has concave and convex parts are measured through comparison with reference dimensions, and to an apparatus for practicing the method.
Heretofore, an absolute measurement method or a comparison measurement method has been employed to measure the dimension between the edges of a concave or convex part of a fine pattern. In the absolute measurement method, a microscope system and a stage on which a specimen is mounted are moved relative to each other in two dimensions. The microscope provides a laser beam so that the edges of the concave or convex part are detected from the laser beam reflected from the part. The movement of the stage with respect to the microscope system, i.e., the distance between the edges of the concave or convex part, is measured by a high-accuracy dimension measuring unit such as a laser interference distance measuring unit.
In the absolute measurement method, the resultant measurement is sufficiently high in accuracy if the distance measuring unit used is high in accuracy. However, the method requires an expensive absolute scale such as the above-described laser interference distance measuring unit, and therefore it is unavoidable that an apparatus for practicing the method is very expensive to manufacture.
On the other hand, a comparison type dimension measuring unit is relatively inexpensive. However, it lacks sufficiently high measurement accuracy. For instance a method is known in the art in which an image enlarged by a microscope system is subjected to optical image slit scanning to effect the measurement. In another method, the enlarged image is applied to a television camera so that it is converted to a video signal for measurement. In the case when the pattern of an IC wafer or the like is imaged, the resultant optical image is low in contrast. Accordingly, in this case, it is difficult for the two methods to obtain photoelectric signals with a high S/N ratio. If an image pickup tube is employed for the television camera, the measurement accuracy is lowered by the image distortion which is inherent with this tube. In the case of a solid state camera using two-dimensional CCD's, the image distortion can be prevented, but it is impossible to perform highly accurate measurement because of the low measurement resolution.